path: root/paramiko/transport.py

# Copyright (C) 2003-2007  Robey Pointer <robeypointer@gmail.com>
#
# This file is part of paramiko.
#
# Paramiko is free software; you can redistribute it and/or modify it under the
# terms of the GNU Lesser General Public License as published by the Free
# Software Foundation; either version 2.1 of the License, or (at your option)
# any later version.
#
# Paramiko is distributed in the hope that it will be useful, but WITHOUT ANY
# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
# A PARTICULAR PURPOSE.  See the GNU Lesser General Public License for more
# details.
#
# You should have received a copy of the GNU Lesser General Public License
# along with Paramiko; if not, write to the Free Software Foundation, Inc.,
# 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA.

"""
Core protocol implementation
"""

import os
import socket
import sys
import threading
import time
import weakref
from hashlib import md5, sha1

import paramiko
from paramiko import util
from paramiko.auth_handler import AuthHandler
from paramiko.ssh_gss import GSSAuth
from paramiko.channel import Channel
from paramiko.common import xffffffff, cMSG_CHANNEL_OPEN, cMSG_IGNORE, \
    cMSG_GLOBAL_REQUEST, DEBUG, MSG_KEXINIT, MSG_IGNORE, MSG_DISCONNECT, \
    MSG_DEBUG, ERROR, WARNING, cMSG_UNIMPLEMENTED, INFO, cMSG_KEXINIT, \
    cMSG_NEWKEYS, MSG_NEWKEYS, cMSG_REQUEST_SUCCESS, cMSG_REQUEST_FAILURE, \
    CONNECTION_FAILED_CODE, OPEN_FAILED_ADMINISTRATIVELY_PROHIBITED, \
    OPEN_SUCCEEDED, cMSG_CHANNEL_OPEN_FAILURE, cMSG_CHANNEL_OPEN_SUCCESS, \
    MSG_GLOBAL_REQUEST, MSG_REQUEST_SUCCESS, MSG_REQUEST_FAILURE, \
    MSG_CHANNEL_OPEN_SUCCESS, MSG_CHANNEL_OPEN_FAILURE, MSG_CHANNEL_OPEN, \
    MSG_CHANNEL_SUCCESS, MSG_CHANNEL_FAILURE, MSG_CHANNEL_DATA, \
    MSG_CHANNEL_EXTENDED_DATA, MSG_CHANNEL_WINDOW_ADJUST, MSG_CHANNEL_REQUEST, \
    MSG_CHANNEL_EOF, MSG_CHANNEL_CLOSE, MIN_PACKET_SIZE, MAX_WINDOW_SIZE, \
    DEFAULT_WINDOW_SIZE, DEFAULT_MAX_PACKET_SIZE
from paramiko.compress import ZlibCompressor, ZlibDecompressor
from paramiko.dsskey import DSSKey
from paramiko.kex_gex import KexGex
from paramiko.kex_group1 import KexGroup1
from paramiko.kex_group14 import KexGroup14
from paramiko.kex_gss import KexGSSGex, KexGSSGroup1, KexGSSGroup14, NullHostKey
from paramiko.message import Message
from paramiko.packet import Packetizer, NeedRekeyException
from paramiko.primes import ModulusPack
from paramiko.py3compat import string_types, long, byte_ord, b
from paramiko.rsakey import RSAKey
from paramiko.ecdsakey import ECDSAKey
from paramiko.server import ServerInterface
from paramiko.sftp_client import SFTPClient
from paramiko.ssh_exception import (SSHException, BadAuthenticationType,
                                    ChannelException, ProxyCommandFailure)
from paramiko.util import retry_on_signal, ClosingContextManager, clamp_value

from Crypto.Cipher import Blowfish, AES, DES3, ARC4
try:
    from Crypto.Util import Counter
except ImportError:
    from paramiko.util import Counter


# for thread cleanup
_active_threads = []

def _join_lingering_threads():
    for thr in _active_threads:
        thr.stop_thread()

import atexit
atexit.register(_join_lingering_threads)


class Transport (threading.Thread, ClosingContextManager):
    """
    An SSH Transport attaches to a stream (usually a socket), negotiates an
    encrypted session, authenticates, and then creates stream tunnels, called
    `channels <.Channel>`, across the session.  Multiple channels can be
    multiplexed across a single session (and often are, in the case of port
    forwardings).
    
    Instances of this class may be used as context managers.
    """
    _PROTO_ID = '2.0'
    _CLIENT_ID = 'paramiko_%s' % paramiko.__version__

    _preferred_ciphers = ('aes128-ctr', 'aes256-ctr', 'aes128-cbc', 'blowfish-cbc',
                          'aes256-cbc', '3des-cbc', 'arcfour128', 'arcfour256')
    _preferred_macs = ('hmac-sha1', 'hmac-md5', 'hmac-sha1-96', 'hmac-md5-96')
    _preferred_keys = ('ssh-rsa', 'ssh-dss', 'ecdsa-sha2-nistp256')
    _preferred_kex =  ( 'diffie-hellman-group14-sha1', 'diffie-hellman-group-exchange-sha1' , 'diffie-hellman-group1-sha1')
    _preferred_compression = ('none',)

    _cipher_info = {
        'aes128-ctr': {'class': AES, 'mode': AES.MODE_CTR, 'block-size': 16, 'key-size': 16},
        'aes256-ctr': {'class': AES, 'mode': AES.MODE_CTR, 'block-size': 16, 'key-size': 32},
        'blowfish-cbc': {'class': Blowfish, 'mode': Blowfish.MODE_CBC, 'block-size': 8, 'key-size': 16},
        'aes128-cbc': {'class': AES, 'mode': AES.MODE_CBC, 'block-size': 16, 'key-size': 16},
        'aes256-cbc': {'class': AES, 'mode': AES.MODE_CBC, 'block-size': 16, 'key-size': 32},
        '3des-cbc': {'class': DES3, 'mode': DES3.MODE_CBC, 'block-size': 8, 'key-size': 24},
        'arcfour128': {'class': ARC4, 'mode': None, 'block-size': 8, 'key-size': 16},
        'arcfour256': {'class': ARC4, 'mode': None, 'block-size': 8, 'key-size': 32},
    }

    _mac_info = {
        'hmac-sha1': {'class': sha1, 'size': 20},
        'hmac-sha1-96': {'class': sha1, 'size': 12},
        'hmac-md5': {'class': md5, 'size': 16},
        'hmac-md5-96': {'class': md5, 'size': 12},
    }

    _key_info = {
        'ssh-rsa': RSAKey,
        'ssh-dss': DSSKey,
        'ecdsa-sha2-nistp256': ECDSAKey,
    }

    _kex_info = {
        'diffie-hellman-group1-sha1': KexGroup1,
        'diffie-hellman-group14-sha1': KexGroup14,
        'diffie-hellman-group-exchange-sha1': KexGex,
        'gss-group1-sha1-toWM5Slw5Ew8Mqkay+al2g==': KexGSSGroup1,
        'gss-group14-sha1-toWM5Slw5Ew8Mqkay+al2g==': KexGSSGroup14,
        'gss-gex-sha1-toWM5Slw5Ew8Mqkay+al2g==': KexGSSGex
    }

    _compression_info = {
        # zlib@openssh.com is just zlib, but only turned on after a successful
        # authentication.  openssh servers may only offer this type because
        # they've had troubles with security holes in zlib in the past.
        'zlib@openssh.com': (ZlibCompressor, ZlibDecompressor),
        'zlib': (ZlibCompressor, ZlibDecompressor),
        'none': (None, None),
    }

    _modulus_pack = None

    def __init__(self,
                 sock,
                 default_window_size=DEFAULT_WINDOW_SIZE,
                 default_max_packet_size=DEFAULT_MAX_PACKET_SIZE,
                 gss_kex=False,
                 gss_deleg_creds=True):
        """
        Create a new SSH session over an existing socket, or socket-like
        object.  This only creates the `.Transport` object; it doesn't begin the
        SSH session yet.  Use `connect` or `start_client` to begin a client
        session, or `start_server` to begin a server session.

        If the object is not actually a socket, it must have the following
        methods:

        - ``send(str)``: Writes from 1 to ``len(str)`` bytes, and returns an
          int representing the number of bytes written.  Returns
          0 or raises ``EOFError`` if the stream has been closed.
        - ``recv(int)``: Reads from 1 to ``int`` bytes and returns them as a
          string.  Returns 0 or raises ``EOFError`` if the stream has been
          closed.
        - ``close()``: Closes the socket.
        - ``settimeout(n)``: Sets a (float) timeout on I/O operations.

        For ease of use, you may also pass in an address (as a tuple) or a host
        string as the ``sock`` argument.  (A host string is a hostname with an
        optional port (separated by ``":"``) which will be converted into a
        tuple of ``(hostname, port)``.)  A socket will be connected to this
        address and used for communication.  Exceptions from the ``socket``
        call may be thrown in this case.

        .. note::
            Modifying the the window and packet sizes might have adverse
            effects on your channels created from this transport. The default
            values are the same as in the OpenSSH code base and have been
            battle tested.

        :param socket sock:
            a socket or socket-like object to create the session over.
        :param int default_window_size:
            sets the default window size on the transport. (defaults to
            2097152)
        :param int default_max_packet_size:
            sets the default max packet size on the transport. (defaults to
            32768)

        .. versionchanged:: 1.15
            Added the ``default_window_size`` and ``default_max_packet_size``
            arguments.
        """
        self.active = False

        if isinstance(sock, string_types):
            # convert "host:port" into (host, port)
            hl = sock.split(':', 1)
            if len(hl) == 1:
                sock = (hl[0], 22)
            else:
                sock = (hl[0], int(hl[1]))
        if type(sock) is tuple:
            # connect to the given (host, port)
            hostname, port = sock
            reason = 'No suitable address family'
            for (family, socktype, proto, canonname, sockaddr) in socket.getaddrinfo(hostname, port, socket.AF_UNSPEC, socket.SOCK_STREAM):
                if socktype == socket.SOCK_STREAM:
                    af = family
                    addr = sockaddr
                    sock = socket.socket(af, socket.SOCK_STREAM)
                    try:
                        retry_on_signal(lambda: sock.connect((hostname, port)))
                    except socket.error as e:
                        reason = str(e)
                    else:
                        break
            else:
                raise SSHException(
                    'Unable to connect to %s: %s' % (hostname, reason))
        # okay, normal socket-ish flow here...
        threading.Thread.__init__(self)
        self.setDaemon(True)
        self.sock = sock
        # Python < 2.3 doesn't have the settimeout method - RogerB
        try:
            # we set the timeout so we can check self.active periodically to
            # see if we should bail.  socket.timeout exception is never
            # propagated.
            self.sock.settimeout(0.1)
        except AttributeError:
            pass

        # negotiated crypto parameters
        self.packetizer = Packetizer(sock)
        self.local_version = 'SSH-' + self._PROTO_ID + '-' + self._CLIENT_ID
        self.remote_version = ''
        self.local_cipher = self.remote_cipher = ''
        self.local_kex_init = self.remote_kex_init = None
        self.local_mac = self.remote_mac = None
        self.local_compression = self.remote_compression = None
        self.session_id = None
        self.host_key_type = None
        self.host_key = None

        # GSS-API / SSPI Key Exchange
        self.use_gss_kex = gss_kex
        # This will be set to True if GSS-API Key Exchange was performed
        self.gss_kex_used = False
        self.kexgss_ctxt = None
        self.gss_host = None
        if self.use_gss_kex:
            self.kexgss_ctxt = GSSAuth("gssapi-keyex", gss_deleg_creds)
            self._preferred_kex = ('gss-gex-sha1-toWM5Slw5Ew8Mqkay+al2g==',
                                   'gss-group14-sha1-toWM5Slw5Ew8Mqkay+al2g==',
                                   'gss-group1-sha1-toWM5Slw5Ew8Mqkay+al2g==',
                                   'diffie-hellman-group-exchange-sha1',
                                   'diffie-hellman-group14-sha1',
                                   'diffie-hellman-group1-sha1')

        # state used during negotiation
        self.kex_engine = None
        self.H = None
        self.K = None

        self.initial_kex_done = False
        self.in_kex = False
        self.authenticated = False
        self._expected_packet = tuple()
        self.lock = threading.Lock()    # synchronization (always higher level than write_lock)

        # tracking open channels
        self._channels = ChannelMap()
        self.channel_events = {}       # (id -> Event)
        self.channels_seen = {}        # (id -> True)
        self._channel_counter = 1
        self.default_max_packet_size = default_max_packet_size
        self.default_window_size = default_window_size
        self._forward_agent_handler = None
        self._x11_handler = None
        self._tcp_handler = None

        self.saved_exception = None
        self.clear_to_send = threading.Event()
        self.clear_to_send_lock = threading.Lock()
        self.clear_to_send_timeout = 30.0
        self.log_name = 'paramiko.transport'
        self.logger = util.get_logger(self.log_name)
        self.packetizer.set_log(self.logger)
        self.auth_handler = None
        self.global_response = None     # response Message from an arbitrary global request
        self.completion_event = None    # user-defined event callbacks
        self.banner_timeout = 15        # how long (seconds) to wait for the SSH banner

        # server mode:
        self.server_mode = False
        self.server_object = None
        self.server_key_dict = {}
        self.server_accepts = []
        self.server_accept_cv = threading.Condition(self.lock)
        self.subsystem_table = {}

    def __repr__(self):
        """
        Returns a string representation of this object, for debugging.
        """
        out = '<paramiko.Transport at %s' % hex(long(id(self)) & xffffffff)
        if not self.active:
            out += ' (unconnected)'
        else:
            if self.local_cipher != '':
                out += ' (cipher %s, %d bits)' % (self.local_cipher,
                                                  self._cipher_info[self.local_cipher]['key-size'] * 8)
            if self.is_authenticated():
                out += ' (active; %d open channel(s))' % len(self._channels)
            elif self.initial_kex_done:
                out += ' (connected; awaiting auth)'
            else:
                out += ' (connecting)'
        out += '>'
        return out

    def atfork(self):
        """
        Terminate this Transport without closing the session.  On posix
        systems, if a Transport is open during process forking, both parent
        and child will share the underlying socket, but only one process can
        use the connection (without corrupting the session).  Use this method
        to clean up a Transport object without disrupting the other process.

        .. versionadded:: 1.5.3
        """
        self.sock.close()
        self.close()

    def get_security_options(self):
        """
        Return a `.SecurityOptions` object which can be used to tweak the
        encryption algorithms this transport will permit (for encryption,
        digest/hash operations, public keys, and key exchanges) and the order
        of preference for them.
        """
        return SecurityOptions(self)

    def set_gss_host(self, gss_host):
        """
        Setter for C{gss_host} if GSS-API Key Exchange is performed.

        :param str gss_host: The targets name in the kerberos database
                             Default: The name of the host to connect to
        :rtype: Void
        """
        # We need the FQDN to get this working with SSPI
        self.gss_host = socket.getfqdn(gss_host)

    def start_client(self, event=None):
        """
        Negotiate a new SSH2 session as a client.  This is the first step after
        creating a new `.Transport`.  A separate thread is created for protocol
        negotiation.

        If an event is passed in, this method returns immediately.  When
        negotiation is done (successful or not), the given ``Event`` will
        be triggered.  On failure, `is_active` will return ``False``.

        (Since 1.4) If ``event`` is ``None``, this method will not return until
        negotation is done.  On success, the method returns normally.
        Otherwise an SSHException is raised.

        After a successful negotiation, you will usually want to authenticate,
        calling `auth_password <Transport.auth_password>` or
        `auth_publickey <Transport.auth_publickey>`.

        .. note:: `connect` is a simpler method for connecting as a client.

        .. note::
            After calling this method (or `start_server` or `connect`), you
            should no longer directly read from or write to the original socket
            object.

        :param .threading.Event event:
            an event to trigger when negotiation is complete (optional)

        :raises SSHException: if negotiation fails (and no ``event`` was passed
            in)
        """
        self.active = True
        if event is not None:
            # async, return immediately and let the app poll for completion
            self.completion_event = event
            self.start()
            return

        # synchronous, wait for a result
        self.completion_event = event = threading.Event()
        self.start()
        while True:
            event.wait(0.1)
            if not self.active:
                e = self.get_exception()
                if e is not None:
                    raise e
                raise SSHException('Negotiation failed.')
            if event.isSet():
                break

    def start_server(self, event=None, server=None):
        """
        Negotiate a new SSH2 session as a server.  This is the first step after
        creating a new `.Transport` and setting up your server host key(s).  A
        separate thread is created for protocol negotiation.

        If an event is passed in, this method returns immediately.  When
        negotiation is done (successful or not), the given ``Event`` will
        be triggered.  On failure, `is_active` will return ``False``.

        (Since 1.4) If ``event`` is ``None``, this method will not return until
        negotation is done.  On success, the method returns normally.
        Otherwise an SSHException is raised.

        After a successful negotiation, the client will need to authenticate.
        Override the methods `get_allowed_auths
        <.ServerInterface.get_allowed_auths>`, `check_auth_none
        <.ServerInterface.check_auth_none>`, `check_auth_password
        <.ServerInterface.check_auth_password>`, and `check_auth_publickey
        <.ServerInterface.check_auth_publickey>` in the given ``server`` object
        to control the authentication process.

        After a successful authentication, the client should request to open a
        channel.  Override `check_channel_request
        <.ServerInterface.check_channel_request>` in the given ``server``
        object to allow channels to be opened.

        .. note::
            After calling this method (or `start_client` or `connect`), you
            should no longer directly read from or write to the original socket
            object.

        :param .threading.Event event:
            an event to trigger when negotiation is complete.
        :param .ServerInterface server:
            an object used to perform authentication and create `channels
            <.Channel>`

        :raises SSHException: if negotiation fails (and no ``event`` was passed
            in)
        """
        if server is None:
            server = ServerInterface()
        self.server_mode = True
        self.server_object = server
        self.active = True
        if event is not None:
            # async, return immediately and let the app poll for completion
            self.completion_event = event
            self.start()
            return

        # synchronous, wait for a result
        self.completion_event = event = threading.Event()
        self.start()
        while True:
            event.wait(0.1)
            if not self.active:
                e = self.get_exception()
                if e is not None:
                    raise e
                raise SSHException('Negotiation failed.')
            if event.isSet():
                break

    def add_server_key(self, key):
        """
        Add a host key to the list of keys used for server mode.  When behaving
        as a server, the host key is used to sign certain packets during the
        SSH2 negotiation, so that the client can trust that we are who we say
        we are.  Because this is used for signing, the key must contain private
        key info, not just the public half.  Only one key of each type (RSA or
        DSS) is kept.

        :param .PKey key:
            the host key to add, usually an `.RSAKey` or `.DSSKey`.
        """
        self.server_key_dict[key.get_name()] = key

    def get_server_key(self):
        """
        Return the active host key, in server mode.  After negotiating with the
        client, this method will return the negotiated host key.  If only one
        type of host key was set with `add_server_key`, that's the only key
        that will ever be returned.  But in cases where you have set more than
        one type of host key (for example, an RSA key and a DSS key), the key
        type will be negotiated by the client, and this method will return the
        key of the type agreed on.  If the host key has not been negotiated
        yet, ``None`` is returned.  In client mode, the behavior is undefined.

        :return:
            host key (`.PKey`) of the type negotiated by the client, or
            ``None``.
        """
        try:
            return self.server_key_dict[self.host_key_type]
        except KeyError:
            pass
        return None

    def load_server_moduli(filename=None):
        """
        (optional)
        Load a file of prime moduli for use in doing group-exchange key
        negotiation in server mode.  It's a rather obscure option and can be
        safely ignored.

        In server mode, the remote client may request "group-exchange" key
        negotiation, which asks the server to send a random prime number that
        fits certain criteria.  These primes are pretty difficult to compute,
        so they can't be generated on demand.  But many systems contain a file
        of suitable primes (usually named something like ``/etc/ssh/moduli``).
        If you call `load_server_moduli` and it returns ``True``, then this
        file of primes has been loaded and we will support "group-exchange" in
        server mode.  Otherwise server mode will just claim that it doesn't
        support that method of key negotiation.

        :param str filename:
            optional path to the moduli file, if you happen to know that it's
            not in a standard location.
        :return:
            True if a moduli file was successfully loaded; False otherwise.

        .. note:: This has no effect when used in client mode.
        """
        Transport._modulus_pack = ModulusPack()
        # places to look for the openssh "moduli" file
        file_list = ['/etc/ssh/moduli', '/usr/local/etc/moduli']
        if filename is not None:
            file_list.insert(0, filename)
        for fn in file_list:
            try:
                Transport._modulus_pack.read_file(fn)
                return True
            except IOError:
                pass
        # none succeeded
        Transport._modulus_pack = None
        return False
    load_server_moduli = staticmethod(load_server_moduli)

    def close(self):
        """
        Close this session, and any open channels that are tied to it.
        """
        if not self.active:
            return
        self.stop_thread()
        for chan in list(self._channels.values()):
            chan._unlink()
        self.sock.close()

    def get_remote_server_key(self):
        """
        Return the host key of the server (in client mode).

        .. note::
            Previously this call returned a tuple of ``(key type, key
            string)``. You can get the same effect by calling `.PKey.get_name`
            for the key type, and ``str(key)`` for the key string.

        :raises SSHException: if no session is currently active.

        :return: public key (`.PKey`) of the remote server
        """
        if (not self.active) or (not self.initial_kex_done):
            raise SSHException('No existing session')
        return self.host_key

    def is_active(self):
        """
        Return true if this session is active (open).

        :return:
            True if the session is still active (open); False if the session is
            closed
        """
        return self.active

    def open_session(self, window_size=None, max_packet_size=None):
        """
        Request a new channel to the server, of type ``"session"``.  This is
        just an alias for calling `open_channel` with an argument of
        ``"session"``.

        .. note:: Modifying the the window and packet sizes might have adverse
            effects on the session created. The default values are the same
            as in the OpenSSH code base and have been battle tested.

        :param int window_size:
            optional window size for this session.
        :param int max_packet_size:
            optional max packet size for this session.

        :return: a new `.Channel`

        :raises SSHException: if the request is rejected or the session ends
            prematurely

        .. versionchanged:: 1.15
            Added the ``window_size`` and ``max_packet_size`` arguments.
        """
        return self.open_channel('session',
                                 window_size=window_size,
                                 max_packet_size=max_packet_size)

    def open_x11_channel(self, src_addr=None):
        """
        Request a new channel to the client, of type ``"x11"``.  This
        is just an alias for ``open_channel('x11', src_addr=src_addr)``.

        :param tuple src_addr:
            the source address (``(str, int)``) of the x11 server (port is the
            x11 port, ie. 6010)
        :return: a new `.Channel`

        :raises SSHException: if the request is rejected or the session ends
            prematurely
        """
        return self.open_channel('x11', src_addr=src_addr)

    def open_forward_agent_channel(self):
        """
        Request a new channel to the client, of type
        ``"auth-agent@openssh.com"``.

        This is just an alias for ``open_channel('auth-agent@openssh.com')``.

        :return: a new `.Channel`

        :raises SSHException:
            if the request is rejected or the session ends prematurely
        """
        return self.open_channel('auth-agent@openssh.com')

    def open_forwarded_tcpip_channel(self, src_addr, dest_addr):
        """
        Request a new channel back to the client, of type ``"forwarded-tcpip"``.
        This is used after a client has requested port forwarding, for sending
        incoming connections back to the client.

        :param src_addr: originator's address
        :param dest_addr: local (server) connected address
        """
        return self.open_channel('forwarded-tcpip', dest_addr, src_addr)

    def open_channel(self,
                     kind,
                     dest_addr=None,
                     src_addr=None,
                     window_size=None,
                     max_packet_size=None):
        """
        Request a new channel to the server. `Channels <.Channel>` are
        socket-like objects used for the actual transfer of data across the
        session. You may only request a channel after negotiating encryption
        (using `connect` or `start_client`) and authenticating.

        .. note:: Modifying the the window and packet sizes might have adverse
            effects on the channel created. The default values are the same
            as in the OpenSSH code base and have been battle tested.

        :param str kind:
            the kind of channel requested (usually ``"session"``,
            ``"forwarded-tcpip"``, ``"direct-tcpip"``, or ``"x11"``)
        :param tuple dest_addr:
            the destination address (address + port tuple) of this port
            forwarding, if ``kind`` is ``"forwarded-tcpip"`` or
            ``"direct-tcpip"`` (ignored for other channel types)
        :param src_addr: the source address of this port forwarding, if
            ``kind`` is ``"forwarded-tcpip"``, ``"direct-tcpip"``, or ``"x11"``
        :param int window_size:
            optional window size for this session.
        :param int max_packet_size:
            optional max packet size for this session.

        :return: a new `.Channel` on success

        :raises SSHException: if the request is rejected or the session ends
            prematurely

        .. versionchanged:: 1.15
            Added the ``window_size`` and ``max_packet_size`` arguments.
        """
        if not self.active:
            raise SSHException('SSH session not active')
        self.lock.acquire()
        try:
            window_size = self._sanitize_window_size(window_size)
            max_packet_size = self._sanitize_packet_size(max_packet_size)
            chanid = self._next_channel()
            m = Message()
            m.add_byte(cMSG_CHANNEL_OPEN)
            m.add_string(kind)
            m.add_int(chanid)
            m.add_int(window_size)
            m.add_int(max_packet_size)
            if (kind == 'forwarded-tcpip') or (kind == 'direct-tcpip'):
                m.add_string(dest_addr[0])
                m.add_int(dest_addr[1])
                m.add_string(src_addr[0])
                m.add_int(src_addr[1])
            elif kind == 'x11':
                m.add_string(src_addr[0])
                m.add_int(src_addr[1])
            chan = Channel(chanid)
            self._channels.put(chanid, chan)
            self.channel_events[chanid] = event = threading.Event()
            self.channels_seen[chanid] = True
            chan._set_transport(self)
            chan._set_window(window_size, max_packet_size)
        finally:
            self.lock.release()
        self._send_user_message(m)
        while True:
            event.wait(0.1)
            if not self.active:
                e = self.get_exception()
                if e is None:
                    e = SSHException('Unable to open channel.')
                raise e
            if event.isSet():
                break
        chan = self._channels.get(chanid)
        if chan is not None:
            return chan
        e = self.get_exception()
        if e is None:
            e = SSHException('Unable to open channel.')
        raise e

    def request_port_forward(self, address, port, handler=None):
        """
        Ask the server to forward TCP connections from a listening port on
        the server, across this SSH session.

        If a handler is given, that handler is called from a different thread
        whenever a forwarded connection arrives.  The handler parameters are::

            handler(channel, (origin_addr, origin_port), (server_addr, server_port))

        where ``server_addr`` and ``server_port`` are the address and port that
        the server was listening on.

        If no handler is set, the default behavior is to send new incoming
        forwarded connections into the accept queue, to be picked up via
        `accept`.

        :param str address: the address to bind when forwarding
        :param int port:
            the port to forward, or 0 to ask the server to allocate any port
        :param callable handler:
            optional handler for incoming forwarded connections, of the form
            ``func(Channel, (str, int), (str, int))``.

        :return: the port number (`int`) allocated by the server

        :raises SSHException: if the server refused the TCP forward request
        """
        if not self.active:
            raise SSHException('SSH session not active')
        port = int(port)
        response = self.global_request('tcpip-forward', (address, port), wait=True)
        if response is None:
            raise SSHException('TCP forwarding request denied')
        if port == 0:
            port = response.get_int()
        if handler is None:
            def default_handler(channel, src_addr, dest_addr_port):
                #src_addr, src_port = src_addr_port
                #dest_addr, dest_port = dest_addr_port
                self._queue_incoming_channel(channel)
            handler = default_handler
        self._tcp_handler = handler
        return port

    def cancel_port_forward(self, address, port):
        """
        Ask the server to cancel a previous port-forwarding request.  No more
        connections to the given address & port will be forwarded across this
        ssh connection.

        :param str address: the address to stop forwarding
        :param int port: the port to stop forwarding
        """
        if not self.active:
            return
        self._tcp_handler = None
        self.global_request('cancel-tcpip-forward', (address, port), wait=True)

    def open_sftp_client(self):
        """
        Create an SFTP client channel from an open transport.  On success, an
        SFTP session will be opened with the remote host, and a new
        `.SFTPClient` object will be returned.

        :return:
            a new `.SFTPClient` referring to an sftp session (channel) across
            this transport
        """
        return SFTPClient.from_transport(self)

    def send_ignore(self, byte_count=None):
        """
        Send a junk packet across the encrypted link.  This is sometimes used
        to add "noise" to a connection to confuse would-be attackers.  It can
        also be used as a keep-alive for long lived connections traversing
        firewalls.

        :param int byte_count:
            the number of random bytes to send in the payload of the ignored
            packet -- defaults to a random number from 10 to 41.
        """
        m = Message()
        m.add_byte(cMSG_IGNORE)
        if byte_count is None:
            byte_count = (byte_ord(os.urandom(1)) % 32) + 10
        m.add_bytes(os.urandom(byte_count))
        self._send_user_message(m)

    def renegotiate_keys(self):
        """
        Force this session to switch to new keys.  Normally this is done
        automatically after the session hits a certain number of packets or
        bytes sent or received, but this method gives you the option of forcing
        new keys whenever you want.  Negotiating new keys causes a pause in
        traffic both ways as the two sides swap keys and do computations.  This
        method returns when the session has switched to new keys.

        :raises SSHException: if the key renegotiation failed (which causes the
            session to end)
        """
        self.completion_event = threading.Event()
        self._send_kex_init()
        while True:
            self.completion_event.wait(0.1)
            if not self.active:
                e = self.get_exception()
                if e is not None:
                    raise e
                raise SSHException('Negotiation failed.')
            if self.completion_event.isSet():
                break
        return

    def set_keepalive(self, interval):
        """
        Turn on/off keepalive packets (default is off).  If this is set, after
        ``interval`` seconds without sending any data over the connection, a
        "keepalive" packet will be sent (and ignored by the remote host).  This
        can be useful to keep connections alive over a NAT, for example.

        :param int interval:
            seconds to wait before sending a keepalive packet (or
            0 to disable keepalives).
        """
        self.packetizer.set_keepalive(interval,
                                      lambda x=weakref.proxy(self): x.global_request('keepalive@lag.net', wait=False))

    def global_request(self, kind, data=None, wait=True):
        """
        Make a global request to the remote host.  These are normally
        extensions to the SSH2 protocol.

        :param str kind: name of the request.
        :param tuple data:
            an optional tuple containing additional data to attach to the
            request.
        :param bool wait:
            ``True`` if this method should not return until a response is
            received; ``False`` otherwise.
        :return:
            a `.Message` containing possible additional data if the request was
            successful (or an empty `.Message` if ``wait`` was ``False``);
            ``None`` if the request was denied.
        """
        if wait:
            self.completion_event = threading.Event()
        m = Message()
        m.add_byte(cMSG_GLOBAL_REQUEST)
        m.add_string(kind)
        m.add_boolean(wait)
        if data is not None:
            m.add(*data)
        self._log(DEBUG, 'Sending global request "%s"' % kind)
        self._send_user_message(m)
        if not wait:
            return None
        while True:
            self.completion_event.wait(0.1)
            if not self.active:
                return None
            if self.completion_event.isSet():
                break
        return self.global_response

    def accept(self, timeout=None):
        """
        Return the next channel opened by the client over this transport, in
        server mode.  If no channel is opened before the given timeout, ``None``
        is returned.

        :param int timeout:
            seconds to wait for a channel, or ``None`` to wait forever
        :return: a new `.Channel` opened by the client
        """
        self.lock.acquire()
        try:
            if len(self.server_accepts) > 0:
                chan = self.server_accepts.pop(0)
            else:
                self.server_accept_cv.wait(timeout)
                if len(self.server_accepts) > 0:
                    chan = self.server_accepts.pop(0)
                else:
                    # timeout
                    chan = None
        finally:
            self.lock.release()
        return chan

    def connect(self, hostkey=None, username='', password=None, pkey=None,
                gss_host=None, gss_auth=False, gss_kex=False, gss_deleg_creds=True):
        """
        Negotiate an SSH2 session, and optionally verify the server's host key
        and authenticate using a password or private key.  This is a shortcut
        for `start_client`, `get_remote_server_key`, and
        `Transport.auth_password` or `Transport.auth_publickey`.  Use those
        methods if you want more control.

        You can use this method immediately after creating a Transport to
        negotiate encryption with a server.  If it fails, an exception will be
        thrown.  On success, the method will return cleanly, and an encrypted
        session exists.  You may immediately call `open_channel` or
        `open_session` to get a `.Channel` object, which is used for data
        transfer.

        .. note::
            If you fail to supply a password or private key, this method may
            succeed, but a subsequent `open_channel` or `open_session` call may
            fail because you haven't authenticated yet.

        :param .PKey hostkey:
            the host key expected from the server, or ``None`` if you don't
            want to do host key verification.
        :param str username: the username to authenticate as.
        :param str password:
            a password to use for authentication, if you want to use password
            authentication; otherwise ``None``.
        :param .PKey pkey:
            a private key to use for authentication, if you want to use private
            key authentication; otherwise ``None``.
        :param str gss_host: The targets name in the kerberos database. default: hostname
        :param bool gss_auth: ``True`` if you want to use GSS-API authentication
        :param bool gss_kex: Perform GSS-API Key Exchange and user authentication
        :param bool gss_deleg_creds: Delegate GSS-API client credentials or not

        :raises SSHException: if the SSH2 negotiation fails, the host key
            supplied by the server is incorrect, or authentication fails.
        """
        if hostkey is not None:
            self._preferred_keys = [hostkey.get_name()]

        self.start_client()

        # check host key if we were given one
        # If GSS-API Key Exchange was performed, we are not required to check
        # the host key.
        if (hostkey is not None) and not gss_kex:
            key = self.get_remote_server_key()
            if (key.get_name() != hostkey.get_name()) or (key.asbytes() != hostkey.asbytes()):
                self._log(DEBUG, 'Bad host key from server')
                self._log(DEBUG, 'Expected: %s: %s' % (hostkey.get_name(), repr(hostkey.asbytes())))
                self._log(DEBUG, 'Got     : %s: %s' % (key.get_name(), repr(key.asbytes())))
                raise SSHException('Bad host key from server')
            self._log(DEBUG, 'Host key verified (%s)' % hostkey.get_name())

        if (pkey is not None) or (password is not None) or gss_auth or gss_kex:
            if gss_auth:
                self._log(DEBUG, 'Attempting GSS-API auth... (gssapi-with-mic)')
                self.auth_gssapi_with_mic(username, gss_host, gss_deleg_creds)
            elif gss_kex:
                self._log(DEBUG, 'Attempting GSS-API auth... (gssapi-keyex)')
                self.auth_gssapi_keyex(username)
            elif pkey is not None:
                self._log(DEBUG, 'Attempting public-key auth...')
                self.auth_publickey(username, pkey)
            else:
                self._log(DEBUG, 'Attempting password auth...')
                self.auth_password(username, password)

        return

    def get_exception(self):
        """
        Return any exception that happened during the last server request.
        This can be used to fetch more specific error information after using
        calls like `start_client`.  The exception (if any) is cleared after
        this call.

        :return:
            an exception, or ``None`` if there is no stored exception.

        .. versionadded:: 1.1
        """
        self.lock.acquire()
        try:
            e = self.saved_exception
            self.saved_exception = None
            return e
        finally:
            self.lock.release()

    def set_subsystem_handler(self, name, handler, *larg, **kwarg):
        """
        Set the handler class for a subsystem in server mode.  If a request
        for this subsystem is made on an open ssh channel later, this handler
        will be constructed and called -- see `.SubsystemHandler` for more
        detailed documentation.

        Any extra parameters (including keyword arguments) are saved and
        passed to the `.SubsystemHandler` constructor later.

        :param str name: name of the subsystem.
        :param class handler:
            subclass of `.SubsystemHandler` that handles this subsystem.
        """
        try:
            self.lock.acquire()
            self.subsystem_table[name] = (handler, larg, kwarg)
        finally:
            self.lock.release()

    def is_authenticated(self):
        """
        Return true if this session is active and authenticated.

        :return:
            True if the session is still open and has been authenticated
            successfully; False if authentication failed and/or the session is
            closed.
        """
        return self.active and (self.auth_handler is not None) and self.auth_handler.is_authenticated()

    def get_username(self):
        """
        Return the username this connection is authenticated for.  If the
        session is not authenticated (or authentication failed), this method
        returns ``None``.

        :return: username that was authenticated (a `str`), or ``None``.
        """
        if not self.active or (self.auth_handler is None):
            return None
        return self.auth_handler.get_username()

    def get_banner(self):
        """
        Return the banner supplied by the server upon connect. If no banner is
        supplied, this method returns C{None}.

        @return: server supplied banner, or C{None}.
        @rtype: string
        """
        if not self.active or (self.auth_handler is None):
            return None
        return self.auth_handler.banner

    def auth_none(self, username):
        """
        Try to authenticate to the server using no authentication at all.
        This will almost always fail.  It may be useful for determining the
        list of authentication types supported by the server, by catching the
        `.BadAuthenticationType` exception raised.

        :param str username: the username to authenticate as
        :return:
            `list` of auth types permissible for the next stage of
            authentication (normally empty)

        :raises BadAuthenticationType: if "none" authentication isn't allowed
            by the server for this user
        :raises SSHException: if the authentication failed due to a network
            error

        .. versionadded:: 1.5
        """
        if (not self.active) or (not self.initial_kex_done):
            raise SSHException('No existing session')
        my_event = threading.Event()
        self.auth_handler = AuthHandler(self)
        self.auth_handler.auth_none(username, my_event)
        return self.auth_handler.wait_for_response(my_event)

    def auth_password(self, username, password, event=None, fallback=True):
        """
        Authenticate to the server using a password.  The username and password
        are sent over an encrypted link.

        If an ``event`` is passed in, this method will return immediately, and
        the event will be triggered once authentication succeeds or fails.  On
        success, `is_authenticated` will return ``True``.  On failure, you may
        use `get_exception` to get more detailed error information.

        Since 1.1, if no event is passed, this method will block until the
        authentication succeeds or fails.  On failure, an exception is raised.
        Otherwise, the method simply returns.

        Since 1.5, if no event is passed and ``fallback`` is ``True`` (the
        default), if the server doesn't support plain password authentication
        but does support so-called "keyboard-interactive" mode, an attempt
        will be made to authenticate using this interactive mode.  If it fails,
        the normal exception will be thrown as if the attempt had never been
        made.  This is useful for some recent Gentoo and Debian distributions,
        which turn off plain password authentication in a misguided belief
        that interactive authentication is "more secure".  (It's not.)

        If the server requires multi-step authentication (which is very rare),
        this method will return a list of auth types permissible for the next
        step.  Otherwise, in the normal case, an empty list is returned.

        :param str username: the username to authenticate as
        :param basestring password: the password to authenticate with
        :param .threading.Event event:
            an event to trigger when the authentication attempt is complete
            (whether it was successful or not)
        :param bool fallback:
            ``True`` if an attempt at an automated "interactive" password auth
            should be made if the server doesn't support normal password auth
        :return:
            `list` of auth types permissible for the next stage of
            authentication (normally empty)

        :raises BadAuthenticationType: if password authentication isn't
            allowed by the server for this user (and no event was passed in)
        :raises AuthenticationException: if the authentication failed (and no
            event was passed in)
        :raises SSHException: if there was a network error
        """
        if (not self.active) or (not self.initial_kex_done):
            # we should never try to send the password unless we're on a secure link
            raise SSHException('No existing session')
        if event is None:
            my_event = threading.Event()
        else:
            my_event = event
        self.auth_handler = AuthHandler(self)
        self.auth_handler.auth_password(username, password, my_event)
        if event is not None:
            # caller wants to wait for event themselves
            return []
        try:
            return self.auth_handler.wait_for_response(my_event)
        except BadAuthenticationType as e:
            # if password auth isn't allowed, but keyboard-interactive *is*, try to fudge it
            if not fallback or ('keyboard-interactive' not in e.allowed_types):
                raise
            try:
                def handler(title, instructions, fields):
                    if len(fields) > 1:
                        raise SSHException('Fallback authentication failed.')
                    if len(fields) == 0:
                        # for some reason, at least on os x, a 2nd request will
                        # be made with zero fields requested.  maybe it's just
                        # to try to fake out automated scripting of the exact
                        # type we're doing here.  *shrug* :)
                        return []
                    return [password]
                return self.auth_interactive(username, handler)
            except SSHException:
                # attempt failed; just raise the original exception
                raise e

    def auth_publickey(self, username, key, event=None):
        """
        Authenticate to the server using a private key.  The key is used to
        sign data from the server, so it must include the private part.

        If an ``event`` is passed in, this method will return immediately, and
        the event will be triggered once authentication succeeds or fails.  On
        success, `is_authenticated` will return ``True``.  On failure, you may
        use `get_exception` to get more detailed error information.

        Since 1.1, if no event is passed, this method will block until the
        authentication succeeds or fails.  On failure, an exception is raised.
        Otherwise, the method simply returns.

        If the server requires multi-step authentication (which is very rare),
        this method will return a list of auth types permissible for the next
        step.  Otherwise, in the normal case, an empty list is returned.

        :param str username: the username to authenticate as
        :param .PKey key: the private key to authenticate with
        :param .threading.Event event:
            an event to trigger when the authentication attempt is complete
            (whether it was successful or not)
        :return:
            `list` of auth types permissible for the next stage of
            authentication (normally empty)

        :raises BadAuthenticationType: if public-key authentication isn't
            allowed by the server for this user (and no event was passed in)
        :raises AuthenticationException: if the authentication failed (and no
            event was passed in)
        :raises SSHException: if there was a network error
        """
        if (not self.active) or (not self.initial_kex_done):
            # we should never try to authenticate unless we're on a secure link
            raise SSHException('No existing session')
        if event is None:
            my_event = threading.Event()
        else:
            my_event = event
        self.auth_handler = AuthHandler(self)
        self.auth_handler.auth_publickey(username, key, my_event)
        if event is not None:
            # caller wants to wait for event themselves
            return []
        return self.auth_handler.wait_for_response(my_event)

    def auth_interactive(self, username, handler, submethods=''):
        """
        Authenticate to the server interactively.  A handler is used to answer
        arbitrary questions from the server.  On many servers, this is just a
        dumb wrapper around PAM.

        This method will block until the authentication succeeds or fails,
        peroidically calling the handler asynchronously to get answers to
        authentication questions.  The handler may be called more than once
        if the server continues to ask questions.

        The handler is expected to be a callable that will handle calls of the
        form: ``handler(title, instructions, prompt_list)``.  The ``title`` is
        meant to be a dialog-window title, and the ``instructions`` are user
        instructions (both are strings).  ``prompt_list`` will be a list of
        prompts, each prompt being a tuple of ``(str, bool)``.  The string is
        the prompt and the boolean indicates whether the user text should be
        echoed.

        A sample call would thus be:
        ``handler('title', 'instructions', [('Password:', False)])``.

        The handler should return a list or tuple of answers to the server's
        questions.

        If the server requires multi-step authentication (which is very rare),
        this method will return a list of auth types permissible for the next
        step.  Otherwise, in the normal case, an empty list is returned.

        :param str username: the username to authenticate as
        :param callable handler: a handler for responding to server questions
        :param str submethods: a string list of desired submethods (optional)
        :return:
            `list` of auth types permissible for the next stage of
            authentication (normally empty).

        :raises BadAuthenticationType: if public-key authentication isn't
            allowed by the server for this user
        :raises AuthenticationException: if the authentication failed
        :raises SSHException: if there was a network error

        .. versionadded:: 1.5
        """
        if (not self.active) or (not self.initial_kex_done):
            # we should never try to authenticate unless we're on a secure link
            raise SSHException('No existing session')
        my_event = threading.Event()
        self.auth_handler = AuthHandler(self)
        self.auth_handler.auth_interactive(username, handler, my_event, submethods)
        return self.auth_handler.wait_for_response(my_event)

    def auth_gssapi_with_mic(self, username, gss_host, gss_deleg_creds):
        """
        Authenticate to the Server using GSS-API / SSPI.

        :param str username: The username to authenticate as
        :param str gss_host: The target host
        :param bool gss_deleg_creds: Delegate credentials or not
        :return: list of auth types permissible for the next stage of
                 authentication (normally empty)
        :rtype: list
        :raise BadAuthenticationType: if gssapi-with-mic isn't
            allowed by the server (and no event was passed in)
        :raise AuthenticationException: if the authentication failed (and no
            event was passed in)
        :raise SSHException: if there was a network error
        """
        if (not self.active) or (not self.initial_kex_done):
            # we should never try to authenticate unless we're on a secure link
            raise SSHException('No existing session')
        my_event = threading.Event()
        self.auth_handler = AuthHandler(self)
        self.auth_handler.auth_gssapi_with_mic(username, gss_host, gss_deleg_creds, my_event)
        return self.auth_handler.wait_for_response(my_event)

    def auth_gssapi_keyex(self, username):
        """
        Authenticate to the Server with GSS-API / SSPI if GSS-API Key Exchange
        was the used key exchange method.

        :param str username: The username to authenticate as
        :param str gss_host: The target host
        :param bool gss_deleg_creds: Delegate credentials or not
        :return: list of auth types permissible for the next stage of
                 authentication (normally empty)
        :rtype: list
        :raise BadAuthenticationType: if GSS-API Key Exchange was not performed
                                      (and no event was passed in)
        :raise AuthenticationException: if the authentication failed (and no
            event was passed in)
        :raise SSHException: if there was a network error
        """
        if (not self.active) or (not self.initial_kex_done):
            # we should never try to authenticate unless we're on a secure link
            raise SSHException('No existing session')
        my_event = threading.Event()
        self.auth_handler = AuthHandler(self)
        self.auth_handler.auth_gssapi_keyex(username, my_event)
        return self.auth_handler.wait_for_response(my_event)

    def set_log_channel(self, name):
        """
        Set the channel for this transport's logging.  The default is
        ``"paramiko.transport"`` but it can be set to anything you want. (See
        the `.logging` module for more info.)  SSH Channels will log to a
        sub-channel of the one specified.

        :param str name: new channel name for logging

        .. versionadded:: 1.1
        """
        self.log_name = name
        self.logger = util.get_logger(name)
        self.packetizer.set_log(self.logger)

    def get_log_channel(self):
        """
        Return the channel name used for this transport's logging.

        :return: channel name as a `str`

        .. versionadded:: 1.2
        """
        return self.log_name

    def set_hexdump(self, hexdump):
        """
        Turn on/off logging a hex dump of protocol traffic at DEBUG level in
        the logs.  Normally you would want this off (which is the default),
        but if you are debugging something, it may be useful.

        :param bool hexdump:
            ``True`` to log protocol traffix (in hex) to the log; ``False``
            otherwise.
        """
        self.packetizer.set_hexdump(hexdump)

    def get_hexdump(self):
        """
        Return ``True`` if the transport is currently logging hex dumps of
        protocol traffic.

        :return: ``True`` if hex dumps are being logged, else ``False``.

        .. versionadded:: 1.4
        """
        return self.packetizer.get_hexdump()

    def use_compression(self, compress=True):
        """
        Turn on/off compression.  This will only have an affect before starting
        the transport (ie before calling `connect`, etc).  By default,
        compression is off since it negatively affects interactive sessions.

        :param bool compress:
            ``True`` to ask the remote client/server to compress traffic;
            ``False`` to refuse compression

        .. versionadded:: 1.5.2
        """
        if compress:
            self._preferred_compression = ('zlib@openssh.com', 'zlib', 'none')
        else:
            self._preferred_compression = ('none',)

    def getpeername(self):
        """
        Return the address of the remote side of this Transport, if possible.
        This is effectively a wrapper around ``'getpeername'`` on the underlying
        socket.  If the socket-like object has no ``'getpeername'`` method,
        then ``("unknown", 0)`` is returned.

        :return:
            the address of the remote host, if known, as a ``(str, int)``
            tuple.
        """
        gp = getattr(self.sock, 'getpeername', None)
        if gp is None:
            return 'unknown', 0
        return gp()

    def stop_thread(self):
        self.active = False
        self.packetizer.close()
        while self.is_alive() and (self is not threading.current_thread()):
            self.join(10)

    ###  internals...

    def _log(self, level, msg, *args):
        if issubclass(type(msg), list):
            for m in msg:
                self.logger.log(level, m)
        else:
            self.logger.log(level, msg, *args)

    def _get_modulus_pack(self):
        """used by KexGex to find primes for group exchange"""
        return self._modulus_pack

    def _next_channel(self):
        """you are holding the lock"""
        chanid = self._channel_counter
        while self._channels.get(chanid) is not None:
            self._channel_counter = (self._channel_counter + 1) & 0xffffff
            chanid = self._channel_counter
        self._channel_counter = (self._channel_counter + 1) & 0xffffff
        return chanid

    def _unlink_channel(self, chanid):
        """used by a Channel to remove itself from the active channel list"""
        self._channels.delete(chanid)

    def _send_message(self, data):
        self.packetizer.send_message(data)

    def _send_user_message(self, data):
        """
        send a message, but block if we're in key negotiation.  this is used
        for user-initiated requests.
        """
        start = time.time()
        while True:
            self.clear_to_send.wait(0.1)
            if not self.active:
                self._log(DEBUG, 'Dropping user packet because connection is dead.')
                return
            self.clear_to_send_lock.acquire()
            if self.clear_to_send.isSet():
                break
            self.clear_to_send_lock.release()
            if time.time() > start + self.clear_to_send_timeout:
                raise SSHException('Key-exchange timed out waiting for key negotiation')
        try:
            self._send_message(data)
        finally:
            self.clear_to_send_lock.release()

    def _set_K_H(self, k, h):
        """used by a kex object to set the K (root key) and H (exchange hash)"""
        self.K = k
        self.H = h
        if self.session_id is None:
            self.session_id = h

    def _expect_packet(self, *ptypes):
        """used by a kex object to register the next packet type it expects to see"""
        self._expected_packet = tuple(ptypes)

    def _verify_key(self, host_key, sig):
        key = self._key_info[self.host_key_type](Message(host_key))
        if key is None:
            raise SSHException('Unknown host key type')
        if not key.verify_ssh_sig(self.H, Message(sig)):
            raise SSHException('Signature verification (%s) failed.' % self.host_key_type)
        self.host_key = key

    def _compute_key(self, id, nbytes):
        """id is 'A' - 'F' for the various keys used by ssh"""
        m = Message()
        m.add_mpint(self.K)
        m.add_bytes(self.H)
        m.add_byte(b(id))
        m.add_bytes(self.session_id)
        out = sofar = sha1(m.asbytes()).digest()
        while len(out) < nbytes:
            m = Message()
            m.add_mpint(self.K)
            m.add_bytes(self.H)
            m.add_bytes(sofar)
            digest = sha1(m.asbytes()).digest()
            out += digest
            sofar += digest
        return out[:nbytes]

    def _get_cipher(self, name, key, iv):
        if name not in self._cipher_info:
            raise SSHException('Unknown client cipher ' + name)
        if name in ('arcfour128', 'arcfour256'):
            # arcfour cipher
            cipher = self._cipher_info[name]['class'].new(key)
            # as per RFC 4345, the first 1536 bytes of keystream
            # generated by the cipher MUST be discarded
            cipher.encrypt(" " * 1536)
            return cipher
        elif name.endswith("-ctr"):
            # CTR modes, we need a counter
            counter = Counter.new(nbits=self._cipher_info[name]['block-size'] * 8, initial_value=util.inflate_long(iv, True))
            return self._cipher_info[name]['class'].new(key, self._cipher_info[name]['mode'], iv, counter)
        else:
            return self._cipher_info[name]['class'].new(key, self._cipher_info[name]['mode'], iv)

    def _set_forward_agent_handler(self, handler):
        if handler is None:
            def default_handler(channel):
                self._queue_incoming_channel(channel)
            self._forward_agent_handler = default_handler
        else:
            self._forward_agent_handler = handler

    def _set_x11_handler(self, handler):
        # only called if a channel has turned on x11 forwarding
        if handler is None:
            # by default, use the same mechanism as accept()
            def default_handler(channel, src_addr_port):
                self._queue_incoming_channel(channel)
            self._x11_handler = default_handler
        else:
            self._x11_handler = handler

    def _queue_incoming_channel(self, channel):
        self.lock.acquire()
        try:
            self.server_accepts.append(channel)
            self.server_accept_cv.notify()
        finally:
            self.lock.release()

    def _sanitize_window_size(self, window_size):
        if window_size is None:
            window_size = self.default_window_size
        return clamp_value(MIN_PACKET_SIZE, window_size, MAX_WINDOW_SIZE)

    def _sanitize_packet_size(self, max_packet_size):
        if max_packet_size is None:
            max_packet_size = self.default_max_packet_size
        return clamp_value(MIN_PACKET_SIZE, max_packet_size, MAX_WINDOW_SIZE)


    def run(self):
        # (use the exposed "run" method, because if we specify a thread target
        # of a private method, threading.Thread will keep a reference to it
        # indefinitely, creating a GC cycle and not letting Transport ever be
        # GC'd. it's a bug in Thread.)

        # Hold reference to 'sys' so we can test sys.modules to detect
        # interpreter shutdown.
        self.sys = sys

        # active=True occurs before the thread is launched, to avoid a race
        _active_threads.append(self)
        if self.server_mode:
            self._log(DEBUG, 'starting thread (server mode): %s' % hex(long(id(self)) & xffffffff))
        else:
            self._log(DEBUG, 'starting thread (client mode): %s' % hex(long(id(self)) & xffffffff))
        try:
            try:
                self.packetizer.write_all(b(self.local_version + '\r\n'))
                self._check_banner()
                self._send_kex_init()
                self._expect_packet(MSG_KEXINIT)

                while self.active:
                    if self.packetizer.need_rekey() and not self.in_kex:
                        self._send_kex_init()
                    try:
                        ptype, m = self.packetizer.read_message()
                    except NeedRekeyException:
                        continue
                    if ptype == MSG_IGNORE:
                        continue
                    elif ptype == MSG_DISCONNECT:
                        self._parse_disconnect(m)
                        self.active = False
                        self.packetizer.close()
                        break
                    elif ptype == MSG_DEBUG:
                        self._parse_debug(m)
                        continue
                    if len(self._expected_packet) > 0:
                        if ptype not in self._expected_packet:
                            raise SSHException('Expecting packet from %r, got %d' % (self._expected_packet, ptype))
                        self._expected_packet = tuple()
                        if (ptype >= 30) and (ptype <= 41):
                            self.kex_engine.parse_next(ptype, m)
                            continue

                    if ptype in self._handler_table:
                        self._handler_table[ptype](self, m)
                    elif ptype in self._channel_handler_table:
                        chanid = m.get_int()
                        chan = self._channels.get(chanid)
                        if chan is not None:
                            self._channel_handler_table[ptype](chan, m)
                        elif chanid in self.channels_seen:
                            self._log(DEBUG, 'Ignoring message for dead channel %d' % chanid)
                        else:
                            self._log(ERROR, 'Channel request for unknown channel %d' % chanid)
                            self.active = False
                            self.packetizer.close()
                    elif (self.auth_handler is not None) and (ptype in self.auth_handler._handler_table):
                        self.auth_handler._handler_table[ptype](self.auth_handler, m)
                    else:
                        self._log(WARNING, 'Oops, unhandled type %d' % ptype)
                        msg = Message()
                        msg.add_byte(cMSG_UNIMPLEMENTED)
                        msg.add_int(m.seqno)
                        self._send_message(msg)
            except SSHException as e:
                self._log(ERROR, 'Exception: ' + str(e))
                self._log(ERROR, util.tb_strings())
                self.saved_exception = e
            except EOFError as e:
                self._log(DEBUG, 'EOF in transport thread')
                #self._log(DEBUG, util.tb_strings())
                self.saved_exception = e
            except socket.error as e:
                if type(e.args) is tuple:
                    if e.args:
                        emsg = '%s (%d)' % (e.args[1], e.args[0])
                    else:  # empty tuple, e.g. socket.timeout
                        emsg = str(e) or repr(e)
                else:
                    emsg = e.args
                self._log(ERROR, 'Socket exception: ' + emsg)
                self.saved_exception = e
            except Exception as e:
                self._log(ERROR, 'Unknown exception: ' + str(e))
                self._log(ERROR, util.tb_strings())
                self.saved_exception = e
            _active_threads.remove(self)
            for chan in list(self._channels.values()):
                chan._unlink()
            if self.active:
                self.active = False
                self.packetizer.close()
                if self.completion_event is not None:
                    self.completion_event.set()
                if self.auth_handler is not None:
                    self.auth_handler.abort()
                for event in self.channel_events.values():
                    event.set()
                try:
                    self.lock.acquire()
                    self.server_accept_cv.notify()
                finally:
                    self.lock.release()
            self.sock.close()
        except:
            # Don't raise spurious 'NoneType has no attribute X' errors when we
            # wake up during interpreter shutdown. Or rather -- raise
            # everything *if* sys.modules (used as a convenient sentinel)
            # appears to still exist.
            if self.sys.modules is not None:
                raise

    ###  protocol stages

    def _negotiate_keys(self, m):
        # throws SSHException on anything unusual
        self.clear_to_send_lock.acquire()
        try:
            self.clear_to_send.clear()
        finally:
            self.clear_to_send_lock.release()
        if self.local_kex_init is None:
            # remote side wants to renegotiate
            self._send_kex_init()
        self._parse_kex_init(m)
        self.kex_engine.start_kex()

    def _check_banner(self):
        # this is slow, but we only have to do it once
        for i in range(100):
            # give them 15 seconds for the first line, then just 2 seconds
            # each additional line.  (some sites have very high latency.)
            if i == 0:
                timeout = self.banner_timeout
            else:
                timeout = 2
            try:
                buf = self.packetizer.readline(timeout)
            except ProxyCommandFailure:
                raise
            except Exception as e:
                raise SSHException('Error reading SSH protocol banner' + str(e))
            if buf[:4] == 'SSH-':
                break
            self._log(DEBUG, 'Banner: ' + buf)
        if buf[:4] != 'SSH-':
            raise SSHException('Indecipherable protocol version "' + buf + '"')
        # save this server version string for later
        self.remote_version = buf
        # pull off any attached comment
        comment = ''
        i = buf.find(' ')
        if i >= 0:
            comment = buf[i+1:]
            buf = buf[:i]
        # parse out version string and make sure it matches
        segs = buf.split('-', 2)
        if len(segs) < 3:
            raise SSHException('Invalid SSH banner')
        version = segs[1]
        client = segs[2]
        if version != '1.99' and version != '2.0':
            raise SSHException('Incompatible version (%s instead of 2.0)' % (version,))
        self._log(INFO, 'Connected (version %s, client %s)' % (version, client))

    def _send_kex_init(self):
        """
        announce to the other side that we'd like to negotiate keys, and what
        kind of key negotiation we support.
        """
        self.clear_to_send_lock.acquire()
        try:
            self.clear_to_send.clear()
        finally:
            self.clear_to_send_lock.release()
        self.in_kex = True
        if self.server_mode:
            if (self._modulus_pack is None) and ('diffie-hellman-group-exchange-sha1' in self._preferred_kex):
                # can't do group-exchange if we don't have a pack of potential primes
                pkex = list(self.get_security_options().kex)
                pkex.remove('diffie-hellman-group-exchange-sha1')
                self.get_security_options().kex = pkex
            available_server_keys = list(filter(list(self.server_key_dict.keys()).__contains__,
                                                self._preferred_keys))
        else:
            available_server_keys = self._preferred_keys

        m = Message()
        m.add_byte(cMSG_KEXINIT)
        m.add_bytes(os.urandom(16))
        m.add_list(self._preferred_kex)
        m.add_list(available_server_keys)
        m.add_list(self._preferred_ciphers)
        m.add_list(self._preferred_ciphers)
        m.add_list(self._preferred_macs)
        m.add_list(self._preferred_macs)
        m.add_list(self._preferred_compression)
        m.add_list(self._preferred_compression)
        m.add_string(bytes())
        m.add_string(bytes())
        m.add_boolean(False)
        m.add_int(0)
        # save a copy for later (needed to compute a hash)
        self.local_kex_init = m.asbytes()
        self._send_message(m)

    def _parse_kex_init(self, m):
        cookie = m.get_bytes(16)
        kex_algo_list = m.get_list()
        server_key_algo_list = m.get_list()
        client_encrypt_algo_list = m.get_list()
        server_encrypt_algo_list = m.get_list()
        client_mac_algo_list = m.get_list()
        server_mac_algo_list = m.get_list()
        client_compress_algo_list = m.get_list()
        server_compress_algo_list = m.get_list()
        client_lang_list = m.get_list()
        server_lang_list = m.get_list()
        kex_follows = m.get_boolean()
        unused = m.get_int()

        self._log(DEBUG, 'kex algos:' + str(kex_algo_list) + ' server key:' + str(server_key_algo_list) +
                  ' client encrypt:' + str(client_encrypt_algo_list) +
                  ' server encrypt:' + str(server_encrypt_algo_list) +
                  ' client mac:' + str(client_mac_algo_list) +
                  ' server mac:' + str(server_mac_algo_list) +
                  ' client compress:' + str(client_compress_algo_list) +
                  ' server compress:' + str(server_compress_algo_list) +
                  ' client lang:' + str(client_lang_list) +
                  ' server lang:' + str(server_lang_list) +
                  ' kex follows?' + str(kex_follows))

        # as a server, we pick the first item in the client's list that we support.
        # as a client, we pick the first item in our list that the server supports.
        if self.server_mode:
            agreed_kex = list(filter(self._preferred_kex.__contains__, kex_algo_list))
        else:
            agreed_kex = list(filter(kex_algo_list.__contains__, self._preferred_kex))
        if len(agreed_kex) == 0:
            raise SSHException('Incompatible ssh peer (no acceptable kex algorithm)')
        self.kex_engine = self._kex_info[agreed_kex[0]](self)

        if self.server_mode:
            available_server_keys = list(filter(list(self.server_key_dict.keys()).__contains__,
                                                self._preferred_keys))
            agreed_keys = list(filter(available_server_keys.__contains__, server_key_algo_list))
        else:
            agreed_keys = list(filter(server_key_algo_list.__contains__, self._preferred_keys))
        if len(agreed_keys) == 0:
            raise SSHException('Incompatible ssh peer (no acceptable host key)')
        self.host_key_type = agreed_keys[0]
        if self.server_mode and (self.get_server_key() is None):
            raise SSHException('Incompatible ssh peer (can\'t match requested host key type)')

        if self.server_mode:
            agreed_local_ciphers = list(filter(self._preferred_ciphers.__contains__,
                                           server_encrypt_algo_list))
            agreed_remote_ciphers = list(filter(self._preferred_ciphers.__contains__,
                                          client_encrypt_algo_list))
        else:
            agreed_local_ciphers = list(filter(client_encrypt_algo_list.__contains__,
                                          self._preferred_ciphers))
            agreed_remote_ciphers = list(filter(server_encrypt_algo_list.__contains__,
                                           self._preferred_ciphers))
        if (len(agreed_local_ciphers) == 0) or (len(agreed_remote_ciphers) == 0):
            raise SSHException('Incompatible ssh server (no acceptable ciphers)')
        self.local_cipher = agreed_local_ciphers[0]
        self.remote_cipher = agreed_remote_ciphers[0]
        self._log(DEBUG, 'Ciphers agreed: local=%s, remote=%s' % (self.local_cipher, self.remote_cipher))

        if self.server_mode:
            agreed_remote_macs = list(filter(self._preferred_macs.__contains__, client_mac_algo_list))
            agreed_local_macs = list(filter(self._preferred_macs.__contains__, server_mac_algo_list))
        else:
            agreed_local_macs = list(filter(client_mac_algo_list.__contains__, self._preferred_macs))
            agreed_remote_macs = list(filter(server_mac_algo_list.__contains__, self._preferred_macs))
        if (len(agreed_local_macs) == 0) or (len(agreed_remote_macs) == 0):
            raise SSHException('Incompatible ssh server (no acceptable macs)')
        self.local_mac = agreed_local_macs[0]
        self.remote_mac = agreed_remote_macs[0]

        if self.server_mode:
            agreed_remote_compression = list(filter(self._preferred_compression.__contains__, client_compress_algo_list))
            agreed_local_compression = list(filter(self._preferred_compression.__contains__, server_compress_algo_list))
        else:
            agreed_local_compression = list(filter(client_compress_algo_list.__contains__, self._preferred_compression))
            agreed_remote_compression = list(filter(server_compress_algo_list.__contains__, self._preferred_compression))
        if (len(agreed_local_compression) == 0) or (len(agreed_remote_compression) == 0):
            raise SSHException('Incompatible ssh server (no acceptable compression) %r %r %r' % (agreed_local_compression, agreed_remote_compression, self._preferred_compression))
        self.local_compression = agreed_local_compression[0]
        self.remote_compression = agreed_remote_compression[0]

        self._log(DEBUG, 'using kex %s; server key type %s; cipher: local %s, remote %s; mac: local %s, remote %s; compression: local %s, remote %s' %
                  (agreed_kex[0], self.host_key_type, self.local_cipher, self.remote_cipher, self.local_mac,
                   self.remote_mac, self.local_compression, self.remote_compression))

        # save for computing hash later...
        # now wait!  openssh has a bug (and others might too) where there are
        # actually some extra bytes (one NUL byte in openssh's case) added to
        # the end of the packet but not parsed.  turns out we need to throw
        # away those bytes because they aren't part of the hash.
        self.remote_kex_init = cMSG_KEXINIT + m.get_so_far()

    def _activate_inbound(self):
        """switch on newly negotiated encryption parameters for inbound traffic"""
        block_size = self._cipher_info[self.remote_cipher]['block-size']
        if self.server_mode:
            IV_in = self._compute_key('A', block_size)
            key_in = self._compute_key('C', self._cipher_info[self.remote_cipher]['key-size'])
        else:
            IV_in = self._compute_key('B', block_size)
            key_in = self._compute_key('D', self._cipher_info[self.remote_cipher]['key-size'])
        engine = self._get_cipher(self.remote_cipher, key_in, IV_in)
        mac_size = self._mac_info[self.remote_mac]['size']
        mac_engine = self._mac_info[self.remote_mac]['class']
        # initial mac keys are done in the hash's natural size (not the potentially truncated
        # transmission size)
        if self.server_mode:
            mac_key = self._compute_key('E', mac_engine().digest_size)
        else:
            mac_key = self._compute_key('F', mac_engine().digest_size)
        self.packetizer.set_inbound_cipher(engine, block_size, mac_engine, mac_size, mac_key)
        compress_in = self._compression_info[self.remote_compression][1]
        if (compress_in is not None) and ((self.remote_compression != 'zlib@openssh.com') or self.authenticated):
            self._log(DEBUG, 'Switching on inbound compression ...')
            self.packetizer.set_inbound_compressor(compress_in())

    def _activate_outbound(self):
        """switch on newly negotiated encryption parameters for outbound traffic"""
        m = Message()
        m.add_byte(cMSG_NEWKEYS)
        self._send_message(m)
        block_size = self._cipher_info[self.local_cipher]['block-size']
        if self.server_mode:
            IV_out = self._compute_key('B', block_size)
            key_out = self._compute_key('D', self._cipher_info[self.local_cipher]['key-size'])
        else:
            IV_out = self._compute_key('A', block_size)
            key_out = self._compute_key('C', self._cipher_info[self.local_cipher]['key-size'])
        engine = self._get_cipher(self.local_cipher, key_out, IV_out)
        mac_size = self._mac_info[self.local_mac]['size']
        mac_engine = self._mac_info[self.local_mac]['class']
        # initial mac keys are done in the hash's natural size (not the potentially truncated
        # transmission size)
        if self.server_mode:
            mac_key = self._compute_key('F', mac_engine().digest_size)
        else:
            mac_key = self._compute_key('E', mac_engine().digest_size)
        sdctr = self.local_cipher.endswith('-ctr')
        self.packetizer.set_outbound_cipher(engine, block_size, mac_engine, mac_size, mac_key, sdctr)
        compress_out = self._compression_info[self.local_compression][0]
        if (compress_out is not None) and ((self.local_compression != 'zlib@openssh.com') or self.authenticated):
            self._log(DEBUG, 'Switching on outbound compression ...')
            self.packetizer.set_outbound_compressor(compress_out())
        if not self.packetizer.need_rekey():
            self.in_kex = False
        # we always expect to receive NEWKEYS now
        self._expect_packet(MSG_NEWKEYS)

    def _auth_trigger(self):
        self.authenticated = True
        # delayed initiation of compression
        if self.local_compression == 'zlib@openssh.com':
            compress_out = self._compression_info[self.local_compression][0]
            self._log(DEBUG, 'Switching on outbound compression ...')
            self.packetizer.set_outbound_compressor(compress_out())
        if self.remote_compression == 'zlib@openssh.com':
            compress_in = self._compression_info[self.remote_compression][1]
            self._log(DEBUG, 'Switching on inbound compression ...')
            self.packetizer.set_inbound_compressor(compress_in())

    def _parse_newkeys(self, m):
        self._log(DEBUG, 'Switch to new keys ...')
        self._activate_inbound()
        # can also free a bunch of stuff here
        self.local_kex_init = self.remote_kex_init = None
        self.K = None
        self.kex_engine = None
        if self.server_mode and (self.auth_handler is None):
            # create auth handler for server mode
            self.auth_handler = AuthHandler(self)
        if not self.initial_kex_done:
            # this was the first key exchange
            self.initial_kex_done = True
        # send an event?
        if self.completion_event is not None:
            self.completion_event.set()
        # it's now okay to send data again (if this was a re-key)
        if not self.packetizer.need_rekey():
            self.in_kex = False
        self.clear_to_send_lock.acquire()
        try:
            self.clear_to_send.set()
        finally:
            self.clear_to_send_lock.release()
        return

    def _parse_disconnect(self, m):
        code = m.get_int()
        desc = m.get_text()
        self._log(INFO, 'Disconnect (code %d): %s' % (code, desc))

    def _parse_global_request(self, m):
        kind = m.get_text()
        self._log(DEBUG, 'Received global request "%s"' % kind)
        want_reply = m.get_boolean()
        if not self.server_mode:
            self._log(DEBUG, 'Rejecting "%s" global request from server.' % kind)
            ok = False
        elif kind == 'tcpip-forward':
            address = m.get_text()
            port = m.get_int()
            ok = self.server_object.check_port_forward_request(address, port)
            if ok:
                ok = (ok,)
        elif kind == 'cancel-tcpip-forward':
            address = m.get_text()
            port = m.get_int()
            self.server_object.cancel_port_forward_request(address, port)
            ok = True
        else:
            ok = self.server_object.check_global_request(kind, m)
        extra = ()
        if type(ok) is tuple:
            extra = ok
            ok = True
        if want_reply:
            msg = Message()
            if ok:
                msg.add_byte(cMSG_REQUEST_SUCCESS)
                msg.add(*extra)
            else:
                msg.add_byte(cMSG_REQUEST_FAILURE)
            self._send_message(msg)

    def _parse_request_success(self, m):
        self._log(DEBUG, 'Global request successful.')
        self.global_response = m
        if self.completion_event is not None:
            self.completion_event.set()

    def _parse_request_failure(self, m):
        self._log(DEBUG, 'Global request denied.')
        self.global_response = None
        if self.completion_event is not None:
            self.completion_event.set()

    def _parse_channel_open_success(self, m):
        chanid = m.get_int()
        server_chanid = m.get_int()
        server_window_size = m.get_int()
        server_max_packet_size = m.get_int()
        chan = self._channels.get(chanid)
        if chan is None:
            self._log(WARNING, 'Success for unrequested channel! [??]')
            return
        self.lock.acquire()
        try:
            chan._set_remote_channel(server_chanid, server_window_size, server_max_packet_size)
            self._log(DEBUG, 'Secsh channel %d opened.' % chanid)
            if chanid in self.channel_events:
                self.channel_events[chanid].set()
                del self.channel_events[chanid]
        finally:
            self.lock.release()
        return

    def _parse_channel_open_failure(self, m):
        chanid = m.get_int()
        reason = m.get_int()
        reason_str = m.get_text()
        lang = m.get_text()
        reason_text = CONNECTION_FAILED_CODE.get(reason, '(unknown code)')
        self._log(ERROR, 'Secsh channel %d open FAILED: %s: %s' % (chanid, reason_str, reason_text))
        self.lock.acquire()
        try:
            self.saved_exception = ChannelException(reason, reason_text)
            if chanid in self.channel_events:
                self._channels.delete(chanid)
                if chanid in self.channel_events:
                    self.channel_events[chanid].set()
                    del self.channel_events[chanid]
        finally:
            self.lock.release()
        return

    def _parse_channel_open(self, m):
        kind = m.get_text()
        chanid = m.get_int()
        initial_window_size = m.get_int()
        max_packet_size = m.get_int()
        reject = False
        if (kind == 'auth-agent@openssh.com') and (self._forward_agent_handler is not None):
            self._log(DEBUG, 'Incoming forward agent connection')
            self.lock.acquire()
            try:
                my_chanid = self._next_channel()
            finally:
                self.lock.release()
        elif (kind == 'x11') and (self._x11_handler is not None):
            origin_addr = m.get_text()
            origin_port = m.get_int()
            self._log(DEBUG, 'Incoming x11 connection from %s:%d' % (origin_addr, origin_port))
            self.lock.acquire()
            try:
                my_chanid = self._next_channel()
            finally:
                self.lock.release()
        elif (kind == 'forwarded-tcpip') and (self._tcp_handler is not None):
            server_addr = m.get_text()
            server_port = m.get_int()
            origin_addr = m.get_text()
            origin_port = m.get_int()
            self._log(DEBUG, 'Incoming tcp forwarded connection from %s:%d' % (origin_addr, origin_port))
            self.lock.acquire()
            try:
                my_chanid = self._next_channel()
            finally:
                self.lock.release()
        elif not self.server_mode:
            self._log(DEBUG, 'Rejecting "%s" channel request from server.' % kind)
            reject = True
            reason = OPEN_FAILED_ADMINISTRATIVELY_PROHIBITED
        else:
            self.lock.acquire()
            try:
                my_chanid = self._next_channel()
            finally:
                self.lock.release()
            if kind == 'direct-tcpip':
                # handle direct-tcpip requests comming from the client
                dest_addr = m.get_text()
                dest_port = m.get_int()
                origin_addr = m.get_text()
                origin_port = m.get_int()
                reason = self.server_object.check_channel_direct_tcpip_request(
                    my_chanid, (origin_addr, origin_port), (dest_addr, dest_port))
            else:
                reason = self.server_object.check_channel_request(kind, my_chanid)
            if reason != OPEN_SUCCEEDED:
                self._log(DEBUG, 'Rejecting "%s" channel request from client.' % kind)
                reject = True
        if reject:
            msg = Message()
            msg.add_byte(cMSG_CHANNEL_OPEN_FAILURE)
            msg.add_int(chanid)
            msg.add_int(reason)
            msg.add_string('')
            msg.add_string('en')
            self._send_message(msg)
            return

        chan = Channel(my_chanid)
        self.lock.acquire()
        try:
            self._channels.put(my_chanid, chan)
            self.channels_seen[my_chanid] = True
            chan._set_transport(self)
            chan._set_window(self.default_window_size, self.default_max_packet_size)
            chan._set_remote_channel(chanid, initial_window_size, max_packet_size)
        finally:
            self.lock.release()
        m = Message()
        m.add_byte(cMSG_CHANNEL_OPEN_SUCCESS)
        m.add_int(chanid)
        m.add_int(my_chanid)
        m.add_int(self.default_window_size)
        m.add_int(self.default_max_packet_size)
        self._send_message(m)
        self._log(DEBUG, 'Secsh channel %d (%s) opened.', my_chanid, kind)
        if kind == 'auth-agent@openssh.com':
            self._forward_agent_handler(chan)
        elif kind == 'x11':
            self._x11_handler(chan, (origin_addr, origin_port))
        elif kind == 'forwarded-tcpip':
            chan.origin_addr = (origin_addr, origin_port)
            self._tcp_handler(chan, (origin_addr, origin_port), (server_addr, server_port))
        else:
            self._queue_incoming_channel(chan)

    def _parse_debug(self, m):
        always_display = m.get_boolean()
        msg = m.get_string()
        lang = m.get_string()
        self._log(DEBUG, 'Debug msg: ' + util.safe_string(msg))

    def _get_subsystem_handler(self, name):
        try:
            self.lock.acquire()
            if name not in self.subsystem_table:
                return None, [], {}
            return self.subsystem_table[name]
        finally:
            self.lock.release()

    _handler_table = {
        MSG_NEWKEYS: _parse_newkeys,
        MSG_GLOBAL_REQUEST: _parse_global_request,
        MSG_REQUEST_SUCCESS: _parse_request_success,
        MSG_REQUEST_FAILURE: _parse_request_failure,
        MSG_CHANNEL_OPEN_SUCCESS: _parse_channel_open_success,
        MSG_CHANNEL_OPEN_FAILURE: _parse_channel_open_failure,
        MSG_CHANNEL_OPEN: _parse_channel_open,
        MSG_KEXINIT: _negotiate_keys,
    }

    _channel_handler_table = {
        MSG_CHANNEL_SUCCESS: Channel._request_success,
        MSG_CHANNEL_FAILURE: Channel._request_failed,
        MSG_CHANNEL_DATA: Channel._feed,
        MSG_CHANNEL_EXTENDED_DATA: Channel._feed_extended,
        MSG_CHANNEL_WINDOW_ADJUST: Channel._window_adjust,
        MSG_CHANNEL_REQUEST: Channel._handle_request,
        MSG_CHANNEL_EOF: Channel._handle_eof,
        MSG_CHANNEL_CLOSE: Channel._handle_close,
    }


class SecurityOptions (object):
    """
    Simple object containing the security preferences of an ssh transport.
    These are tuples of acceptable ciphers, digests, key types, and key
    exchange algorithms, listed in order of preference.

    Changing the contents and/or order of these fields affects the underlying
    `.Transport` (but only if you change them before starting the session).
    If you try to add an algorithm that paramiko doesn't recognize,
    ``ValueError`` will be raised.  If you try to assign something besides a
    tuple to one of the fields, ``TypeError`` will be raised.
    """
    #__slots__ = [ 'ciphers', 'digests', 'key_types', 'kex', 'compression', '_transport' ]
    __slots__ = '_transport'

    def __init__(self, transport):
        self._transport = transport

    def __repr__(self):
        """
        Returns a string representation of this object, for debugging.
        """
        return '<paramiko.SecurityOptions for %s>' % repr(self._transport)

    def _get_ciphers(self):
        return self._transport._preferred_ciphers

    def _get_digests(self):
        return self._transport._preferred_macs

    def _get_key_types(self):
        return self._transport._preferred_keys

    def _get_kex(self):
        return self._transport._preferred_kex

    def _get_compression(self):
        return self._transport._preferred_compression

    def _set(self, name, orig, x):
        if type(x) is list:
            x = tuple(x)
        if type(x) is not tuple:
            raise TypeError('expected tuple or list')
        possible = list(getattr(self._transport, orig).keys())
        forbidden = [n for n in x if n not in possible]
        if len(forbidden) > 0:
            raise ValueError('unknown cipher')
        setattr(self._transport, name, x)

    def _set_ciphers(self, x):
        self._set('_preferred_ciphers', '_cipher_info', x)

    def _set_digests(self, x):
        self._set('_preferred_macs', '_mac_info', x)

    def _set_key_types(self, x):
        self._set('_preferred_keys', '_key_info', x)

    def _set_kex(self, x):
        self._set('_preferred_kex', '_kex_info', x)

    def _set_compression(self, x):
        self._set('_preferred_compression', '_compression_info', x)

    ciphers = property(_get_ciphers, _set_ciphers, None,
                       "Symmetric encryption ciphers")
    digests = property(_get_digests, _set_digests, None,
                       "Digest (one-way hash) algorithms")
    key_types = property(_get_key_types, _set_key_types, None,
                         "Public-key algorithms")
    kex = property(_get_kex, _set_kex, None, "Key exchange algorithms")
    compression = property(_get_compression, _set_compression, None,
                           "Compression algorithms")


class ChannelMap (object):
    def __init__(self):
        # (id -> Channel)
        self._map = weakref.WeakValueDictionary()
        self._lock = threading.Lock()

    def put(self, chanid, chan):
        self._lock.acquire()
        try:
            self._map[chanid] = chan
        finally:
            self._lock.release()

    def get(self, chanid):
        self._lock.acquire()
        try:
            return self._map.get(chanid, None)
        finally:
            self._lock.release()

    def delete(self, chanid):
        self._lock.acquire()
        try:
            try:
                del self._map[chanid]
            except KeyError:
                pass
        finally:
            self._lock.release()

    def values(self):
        self._lock.acquire()
        try:
            return list(self._map.values())
        finally:
            self._lock.release()

    def __len__(self):
        self._lock.acquire()
        try:
            return len(self._map)
        finally:
            self._lock.release()